1. Field of the Invention
The present invention is directed to a graded start rod used to prepare glass preforms which are subsequently drawn into optical waveguides.
2. Description of the Prior Art
During the past decade, a great deal of work has been undertaken directed to the manufacture of glass transmission lines for visible and near-visible electromagnetic radiation, commonly referred to as optical waveguides. In brief, optical waveguides are extremely thin, flexible glass fibers, prepared by drawing glass preforms under controlled conditions. Although these conditions are subject to wide variation, depending upon such factors as the composition of the glass and the drawing procedure, the optical waveguide so prepared will comprise, in its simplest form, a glass core of a given, constant diameter, surrounded by a glass cladding having an index of refraction less than that of the glass core. The cladding acts as a barrier which confines the light to the core.
While the drawing procedure is important in obtaining a properly functioning optical waveguide, it is the production of the glass preform which is drawn into the fiber which controls the ultimate composition and hence transmission properties of the waveguide. While numerous methods have been proposed for preparing such preforms, their goal is the same, i.e., the production of a structure having a core area of a given refractive index, surrounded by a cladding having a lower refractive index to inhibit the attenuation of light being transmitted through the core.
In particular, two types of construction have been suggested. The simplest comprises a cylindrical core having a constant index of refraction, surrounded by a cladding of a constant, but lower, index of refraction. The structure may be modified by addition of other cladding layers of progressively lower, constant indices of refraction to form a step-like construction. The second type of preform construction comprises a core area of graded index of refraction, surrounded by a cladding having a refractive index which is constant and lower than the refractive index of the core.
The manufacture of optical waveguide preforms are accomplished in a number of ways. For example one method, described in U.S. Pat. No. 3,932,160, accomplishes deposition of a borosilicate layer on a pure silica start rod by high temperature pyrolysis of a mixture of silane or silicon halides and borane. This method produces an acicular deposit of borosilicate glass which exhibits a lower index of refraction than the pure silica core.
The methods for manufacturing optical waveguide preforms by forming a cladding on a start rod of uniform composition suffer a common deficiency. As the diameter of the start rod must be sufficiently wide, generally about 3 mm, to insure that it is capable of withstanding the stress of processing, it forms a relatively large central area having a constant index of refraction. This central area remains proportionally large when the preform is drawn into optical waveguides. However, it has been observed that, in order to minimize dispersion of light passing through the core of optical waveguides, it is desirable to minimize the size of the central area in which the index of refraction is constant. Ideally, a waveguide having a core which exhibits a radially decreasing index of refraction is preferred.